1. Field of the Invention
The present invention relates to corrosion-resistant amorphous surface alloys applied onto inexpensive and less corrosion-resistant substrate metallic materials, and also to a process for their preparation. Such surface alloys are suitable for corrosion-resistant materials in extremely aggressive environments such as strong acids including hydrochloric acid in which conventional corrosion-resistant metallic materials such as stainless steels and corrosion resistant nickel alloys induce severe corrosion.
2. Prior Art
Conventional corrosion-resistant iron-base and nickel-base alloys such as Type 304 and Type 316 steels, Carpenter-20, Inconel 600 and Hastelloy C possess excellent corrosion resistance and have been used widely in corrosive environments including chemical plants and the like. However, stainless steels of not high contents of nickel, such as Type 304 and Type 316 steels, even if added with molybdenum suffer pitting and crevice corrosion in aggressive environments containing halogen ions and hence are used in relatively mildly corrosive environments. This problem is also serious in terms of safety and economy as even high nickel alloys including Inconel 600 suffer pitting, crevice corrosion and wall thinning due to general corrosion in aggressive corrosive environments.
Usually, solid alloys are in a crystalline state. However, if a specifically composed alloy is solidified as by rapid quenching from its liquid state, by which the formation of the long range order in atomic arrangement is prevented, an alloy of an amorphous structure similar to the liquid structure can be obtained which is called an amorphous alloy. Most amorphous alloys are homogeneous, single-phase alloys of supersaturated solid solutions. These alloys therefore have remarkably high mechanical strength compared with conventional practical metals and also extraordinarily high corrosion resistance and various other unique characteristics depending on the chemical compositions. The present inventors have studied corrosion-resistant amorphous alloys of such characteristics and have found certain corrosion-resistant amorphous alloys unsusceptible to pitting, crevice and general corrosion in highly aggressive aqueous solutions such as of strong acids and solutions containing high concentrations of chloride ions (Japanese Patent Publication No. 59-50745).
This invention consists of the following: (1) a corrosion-resistant amorphous alloy composed of 5-40 at % Cr and 15-35 at % P and the reminder being Ni, unsusceptible to pitting crevice and general corrosion in aggressively corrosive environments, (2) a corrosion-resistant amorphous alloy composed of 5-40 at % Cr, 15-35 at % P and at least one element selected from 3 at % or less Al, 10 at % or less Mo and less than 40 at % Fe, and the remainder being Ni and the sum of Cr, P, Al, Mo and Fe being less than 60 at %, unsusceptible to pitting, crevice and general corrosion in aggressively corrosive environments, (3) a corrosion-resistant amorphous alloy composed of 5-40 at % Cr, 15-35 at % in the sum of 5-35 at % P and at least one element selected from C, Si and B, and the remainder being Ni, unsusceptible to pitting, crevice and general corrosion in aggressively corrosive environments, and (4) a corrosion-resistant amorphous alloy composed of 5-40 at % Cr, 15-35 at % in the sum of 5-35 at % P and at least one element selected from C, Si and B and at least one element selected from 3 at % or less Al, 10 at % or less Mo and less than 40 at % Fe, and the remainder being Ni, the sum of Cr, P, C, Si, B, Al, Mo and Fe being less than 60 at %, unsusceptible to pitting, crevice and general corrosion in aggressively corrosion environments.
However, as will be described later, the amorphous alloys if prepared by rapid quenching from their liquid state are generally in the form of thin sheet of 200 .mu.m or less because of requirement of rapid absorption of heat of melt. Furthermore, conventional welding methods cannot be applied to them. These facts restrict their practical utilization.
On the other hand, some of the present inventors have previously found that if a metal surface is irradiated with a high energy density beam for a short time for instantaneous melting of a limited volume, the heat of the molten portion is rapidly absorbed by the surrounding solid metal and that this phenomenon can be used to prepare a rapidly solidified amorphous surface alloy onto conventional bulk metals (Japanese Patent Laid-open Publication Nos. 57-155363 and 60-238489).
Certain corrosion-resistant amorphous nickel-base alloys are known as having high corrosion resistance in aggressive environments, but have not so far been used practically. The thickness of amorphous alloys prepared by rapid quenching from the liquid state is generally 10 .mu.m, at most 200 .mu.m, owing to the need to rapidly absorb heat from the melt. Furthermore, the amorphous alloys are thermodynamically metastable and lose their inherent characteristics by crystallization when heated at temperatures higher than the crystallization temperatures, precluding the use of conventional welding methods which require alloys to be heated. The fact that thin amorphous alloys of several tens of microns cannot be welded conventionally is the most serious problem preventing practical utilization of corrosion-resistance amorphous nickel-base alloys.
On the other hand, the extremely high corrosion resistance of corrosion-resistant amorphous nickel-base alloys is required not for bulk structural materials but for the material surface.